Vanadium-Doped NiCoP Nanosheets for Enhanced Hydrogen Evolution Reaction in Alkaline Media

Augmented electronic metal-support interaction of single-atomic NiNC supported PtRu nanoalloys and their boosted activity and durability for hydrogen evolution and oxygen reduction reactions in both alkaline and acidic media.

Boosted hydrogen evolution in alkaline media enabled by a facile oxidation-involving surface modification

In this work, 3D nickel-manganese (NiMn) bimetallic coatings have been studied as electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline (1.0 M KOH) media and the HER in acidic (0.5 M H2SO4) media. The catalysts have been deposited on a titanium substrate (1 × 1 cm2) using low-cost and facile electrochemical deposition method through a dynamic hydrogen bubble template technique. The electrocatalytic performance of these fabricated catalysts was investigated by using Linear Sweep Voltammetry (LSV) for HER and OER at different temperatures ranging from 25 up to 75 °C and also was characterized by scanning electron microscopy (SEM) and inductively coupled plasma optical emission spectroscopy (ICP-OES). It was found that fabricated NiMn/Ti-5 electrocatalyst with Ni2+/Mn2+ molar ratio of 1:5 exhibits excellent HER activity in alkaline media with overpotential of 127.1 mV to reach current density of 10 mA cm−2. On the contrary, NiMn/Ti-1 electrocatalyst that fabricated with Ni2+/Mn2+ molar proportion of 1:1 and lowest Mn-loading of 13.43 µgcm−2 demonstrates exceptional OER activity with minimum overpotential of 356.3 mV to reach current density of 10 mA cm−2. The current densities increase ca. 1.8–2.2 times with an increase in temperature from 25 °C to 75 °C for both HER and OER investigation. Both catalysts also have exhibited excellent long-term stability for 10 h at constant potentials as well as constant current density of 10 mA cm−2 that assure their robustness and higher durability regarding alkaline water splitting.

Self-supported electrocatalysts are directly employed as electrodes for water splitting. Herein, we report an effective strategy to develop flower-on-sheet structured nanohybrids, where CoP nanoflowers are epitaxially grown along the edges of Ni-Co-P nanosheets (namely NiCoP) on carbon cloth (m-CoP–NiCoP/CC), thus obtained as efficient self-supported electrodes for the hydrogen evolution reaction (HER) in both acidic and alkaline media. This unique nanostructure endows NiCoP nanosheets with maximal exposed surface area, along with increased active sites brought by CoP nanoflowers. Moreover, due to good electrical connection between CoP nanoflowers and NiCoP, and between conductive NiCoP and carbon cloth, electrons can easily transfer from active sites to the conductive substrates. Therefore, the m-CoP–NiCoP/CC exhibits superior catalytic activity and stability for HER in both acidic and alkaline media. The as-prepared electrocatalyst requires overpotentials of only 75.0–81.5 mV to deliver a benchmark current density of 10 mA cm−2 in acidic and alkaline media, respectively, which are superior to most of the previously reported metal phosphides-based electrocatalysts. Hence, this work can provide a design for developing highly active electrocatalysts for water splitting. The self-supported CoP-NiCoP nanohybrids were prepared as efficient and durable electrocatalysts for hydrogen evolution reaction in both acidic and alkaline media, where CoP nanoflowers are epitaxially grown along the edges of NiCoP nanosheets.

Developing an efficient catalytic system for electrolysis with reduced platinum (Pt) loading while maintaining performance comparable to bulk platinum metal is important to decrease costs and improve scalability of the hydrogen fuel economy. Here we report the performance of a novel sputter-deposited molybdenum (Mo) thin film with an extremely low co-loading of Pt, where Pt atoms were dispersed on Mo (Ptd-Mo) as an electrocatalyst for the hydrogen evolution reaction (HER) in either alkaline or acidic media. The Ptd-Mo electrocatalyst presents similar catalytic activity to bulk Pt in alkaline media, while the performance is only slightly decreased in acidic media. Differential electrochemical mass spectrometry (DEMS) results confirm that the Ptd-Mo electrocatalyst produced hydrogen at a rate comparable with that of a pristine Pt sample at the same potential. A comparison with Pt-loaded degenerately doped p-type doped silicon (Ptd-Si) suggests that Mo and Pt work synergistically to boost the performance of Ptd-Mo catalysts. Cyclic voltammetry (CV) and X-ray photoelectron spectroscopy (XPS) before and after 1000 cycles of continuous operation confirm the significant durability of the Ptd-Mo performance. Overall, the Ptd-Mo electrocatalyst, with comparable HER activity to bulk Pt despite an ultra-low Pt loading, could be a strong candidate for hydrogen production in either acidic or basic conditions.

Incorporating metal Co into CoMoO4/Co2Mo3O8 heterointerfaces with rich-oxygen vacancies for efficient hydrogen evolution catalysis

Pursuing efficient and low-cost electrocatalysts is crucial for the performance of water–alkali electrolyzers toward water splitting. Earth-abundant transition-metal oxides, in spite of their alluring performances in the oxygen evolution reaction, are thought to be inactive in the hydrogen evolution reaction in alkaline media. Here, we demonstrate that pure TiO2 single crystals, a typical transition-metal oxide, can be activated toward electrocatalytic hydrogen evolution reaction in alkaline media through engineering interfacial oxygen vacancies. Experimental and theoretical results indicate that subsurface oxygen vacancies and low-coordinated Ti ions (Ti3+) can enhance the electrical conductivity and promote electron transfer and hydrogen desorption, which activate reduced TiO2 single crystals in the hydrogen evolution reaction in alkaline media. This study offers a rational route for developing reduced transition-metal oxides for low-cost and highly active hydrogen evolution reaction catalysts, to realize overall water splitting in alkaline media.

Nickel (Ni)-based materials are among the most promising platinum group metal-free (PGM-free) electrocatalysts for hydrogen evolution reaction (HER) in alkaline media. With the growing demand for green hydrogen production, the HER activity of Ni-based catalysts must be enhanced to meet the goal of low-cost and sustainable hydrogen production.1 Carbon-supported Ni-based catalysts are typically prepared through the heat-treatment of Ni-based precursors with carbon supports.2 However, the catalysts obtained by this approach suffer from high content of inactive carbon and a non-uniform distribution of Ni-based nanoparticles, leading to low HER activity. These issues amplify the need for novel Ni-based catalysts with a well-controlled catalyst structure designed for maximizing catalyst utilization and minimizing electrode thickness.In this talk, we will present our recent progress in developing Ni-based HER catalysts derived from metal-organic framework (MOF) precursors. Taking advantage of the characteristics of well-defined structure, high porosity, and flexible chemistry of MOF precursors3, the MOF-derived catalysts exhibit very promising activity towards HER allowing to reach 10 mA/cm2 at an overpotential of less than 100 mV in alkaline media. We will discuss how the ligand in MOF precursors and heat treatment conditions influence the structure and HER performance of Ni-based catalysts. This understanding promises to provide much needed guidance for the advancement of Ni-based catalysts for HER in alkaline media. Acknowledgement This work has been partially supported by the Center for Alkaline Based Energy Solutions (CABES) funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award DE-SC0019445.

It is experimentally challenging to deconvolute the potential‐dependent adsorption of the different intermediates that occur during the hydrogen evolution reaction (HER) in alkaline media. This difficulty has limited our understanding regarding why the HER kinetics are more sluggish in alkaline media compared to acidic media. Herein, we utilized the surface interrogation mode of scanning electrochemical microscopy (SI‐SECM) to investigate the surface adsorbed species that form during the HER in alkaline media on polycrystalline platinum, Pt(poly). To deconvolute the different adsorbed intermediates, we developed a detailed COMSOL‐based kinetic model to rapidly simulate the SI‐SECM titration reactions under our experimental conditions. Utilization of this rapid‐kinetic model overcomes the limitation of SI‐SECM not having the ability to simultaneously resolve multiple surface adsorbed intermediates. We demonstrate that these numerical simulations can separate the potential dependant formation of the underpotential deposition of hydrogen (H(UPD)) from the overpotential deposition (H(OPD)) of hydrogen. In addition, our simulations show that a spectator species may also exist on the surface during HER potentials. Our simulations also show that at full H2‐producing potentials, the surface of Pt(poly) is fully saturated with intermediates. Comparison between the potential‐dependent adsorption of H(OPD) and Tafel analysis reveal that the Heyrovsky step is likely rate‐determining in alkaline media. However, in alkaline media the Heyrovsky step transitions from first‐order in H(OPD) at low H(OPD) coverage to zero‐order at high H(OPD) coverage, due to surface saturation of adsorbed intermediates. Tafel analysis in acidic media shows that the Heyrovsky step is likely rate‐determining, but remains first‐order in Had over a larger potential range. These fundamental insights reveal that a sluggish Heyrovsky step is a major contributor to the attenuated kinetics of the HER in alkaline media.

Metal/antiperovskite metal nitride composites Ag/AgNNi3 as novel efficient electrocatalysts for hydrogen evolution reaction in alkaline media

Hydrogen evolution reaction (HER) plays a vital role in renewable energy conversion for the development of hydrogen-based energy sources. Lately, heterostructures through hybridizing MXenes with two-dimensional materials have been successfully fabricated and attract much attention due to the exceptional performance as electrodes for Li ion storage and electrocatalysts for HER. Herein, we constructed heterostructures of CoNx-graphene (CoNx-G, x = 2 and 4) supported by MXenes (Ti3C2F2 and Ti3C2O2) monolayer as highly active electrocatalysts for HER. The theoretical results show that the CoN2-G/Ti3C2O2 heterostructure exhibits a high performance for HER with an over-potential (Ƞ) of only 0.33 V, and the rate-limiting step is determined to be the initial water dissociation process in alkaline media. The outstanding performance of CoN2-G/Ti3C2O2 is strongly attributed to the interfacial coupling between CoN2-G and the MXene substrate. Our finding demonstrates that the sluggish hydrogen evolution process in alkaline media can be facilitated by taking advantage of the fast charge transfer kinetics and interfacial coupling of MXenes. Herein, we theoretically design and explore 2D hybrid materials of CoNx–G supported by MXene monolayers as highly active HER electrocatalysts by using first-principles calculations. The results show that the CoN2–G/Ti3C2O2 heterostructure has an outstanding HER performance with ΔGH* (0.21 eV) approaching zero as well as water molecule dissociation barrier (ΔGH–OH) of 0.30 eV in alkaline media. This exceptional performance is strongly attributed to the interfacial coupling between CoN2–G and the MXene substrate.

Molybdenum carbide nanosheets decorated with Ni(OH)2 nanoparticles toward efficient hydrogen evolution reaction in alkaline media

Efficient and low-cost electrocatalysts for the hydrogen evolution reaction (HER) in alkaline media are essential for sustainable hydrogen production. In this study, Ni electrocatalysts were deposited on pencil graphite using a simple one-step pulsed current electrodeposition method, from both acidic Watts and alkaline citrate baths. The influence of bath type and electrodeposition parameters—current density and temperature—on catalyst morphology and performance for HER was systematically investigated by scanning electron microscopy and electrochemical methods. Linear sweep voltammetry, chronopotentiometry, and electrochemical impedance spectroscopy (EIS) were used to evaluate the electrocatalytic activity, stability, and HER mechanism. The best catalytic performance was achieved for the Ni electrocatalyst deposited from the citrate bath at 50 mA cm−2 and 40 °C, showing an exchange current density of 0.93 mA cm−2, a Tafel slope of −208 mV dec−1, and overpotentials of −210 mV and −386 mV at 10 and 100 mA cm−2, respectively, in 1 M KOH solution. Chronopotentiometry confirmed improved stability and an overpotential reduction of approximately 92 mV as compared to pure Ni, while EIS revealed the lowest charge transfer resistance. It was shown that the electrocatalysts deposited from the citrate bath outperform those from the Watts bath, and electrodeposition at 40 °C is optimal for achieving the highest electrocatalytic activity for HER.

The 2 orders of magnitude loss of Platinum Group Metal (PGM) activity toward Hydrogen Oxidation and Reduction reactions (HOR/HER) has hindered implementation of alkaline based electrolyzers and fuel cells and demonstrated a significant knowledge gap in our fundamental understanding of electrochemical interfaces.1 Beneath the seemingly simple reaction lies a potentially convoluted mechanism, with various researchers assigning the activity loss to shifts in electrode potential of zero free charge,2 changed binding energies of adsorbates and reactive intermediates,3,4 and the re-arrangement of interfacial water.5 Recently, Monteiro et al. showed an activity dependence on interfacial cation concentration and identity, suggesting that charge dense cations are able to better stabilize the OH- produced from the unfavorable but necessary water splitting in the HER direction near the negatively charged surface.6,7 In this work, we challenge this notion by demonstrating that competition for water molecules between Outer Helmholtz Plane (OHP) cations and the interfacial water structure is responsible for the activity loss. Through the use of traditional Rotating Disk Electrode, in situ Attenuated Total Reflection-Surface Enhanced Infrared Reflection Absorption Spectroscopy (ATR-SEIRAS) and judicious choice of crown ether additives to hinder the water-interfacial cation interaction, we show a positive correlation between the strength and number of interfacial water-water hydrogen bonds and platinum’s HOR/HER activity.The typical double layer structure in an aqueous electrochemical system consists of specifically adsorbed species in the Inner Helmholtz Plane (IHP), the first layer of non-adsorbates at the OHP, and water hydrating the charged species. ATR-SEIRAS has previously been used to analyze the structure of these hydration shells on a CO covered Pt surface, demonstrating hydrating waters interacting mainly with alkali metal cations through the increase of a 3600 cm-1 ν(O-H) stretch and weak interactions with surrounding interfacial water.8 Through chelation with crown ether, these hydration shells convert from strongly to weakly cation interacting, favoring instead hydrogen bonding with other water molecules, shown with increasing absorption bands at lower wavenumbers (3400 cm-1). This transition is accompanied by an increase in 3000 cm-1 on the CO-free Pt surface, previously assigned to strongly hydrogen bonded “ice-like” interfacial water.9 We further show a similar increase in the 3000 cm-1 band for other kinetic enhancing additives recently reported in the literature, supporting the notion that increasing water-water hydrogen bonding is the unifying descriptor for promoting a rigid interfacial “ice-like” structure and improving PGM HOR/HER activity. Durst, J. et al. New insights into the electrochemical hydrogen oxidation and evolution reaction mechanism. Energy Environ. Sci. 7, 2255–2260 (2014).Sarabia, F. J., Sebastián-Pascual, P., Koper, M. T. M., Climent, V. & Feliu, J. M. Effect of the Interfacial Water Structure on the Hydrogen Evolution Reaction on Pt(111) Modified with Different Nickel Hydroxide Coverages in Alkaline Media. ACS Appl. Mater. Interfaces 11, 613–623 (2019).McCrum, I. T. & Koper, M. T. M. The role of adsorbed hydroxide in hydrogen evolution reaction kinetics on modified platinum. Nat. Energy 5, 891–899 (2020).Yang, X., Nash, J., Oliveira, N. J., Yan, Y. & Xu, B. Understanding the pH Dependence of Underpotential Deposited Hydrogen on Platinum. Angew. Chemie - Int. Ed. 58, 17718–17723 (2019).Zhao, K. et al. Enhancing Hydrogen Oxidation and Evolution Kinetics by Tuning the Interfacial Hydrogen-Bonding Environment on Functionalized Platinum Surfaces. Angew. Chemie - Int. Ed. 61, (2022).Monteiro, M. C. O., Goyal, A., Moerland, P. & Koper, M. T. M. Understanding Cation Trends for Hydrogen Evolution on Platinum and Gold Electrodes in Alkaline Media. ACS Catal. 11, 14328–14335 (2021).Goyal, A. & Koper, M. T. M. The Interrelated Effect of Cations and Electrolyte pH on the Hydrogen Evolution Reaction on Gold Electrodes in Alkaline Media. Angew. Chemie - Int. Ed. 60, 13452–13462 (2021).Yamakata, A. & Osawa, M. Cation-dependent restructure of the electric double layer on CO-covered Pt electrodes: Difference between hydrophilic and hydrophobic cations. J. Electroanal. Chem. 800, 19–24 (2017).Osawa, M., Tsushima, M., Mogami, H., Samjeské, G. & Yamakata, A. Structure of water at the electrified platinum-water interface: A study by surface-enhanced infrared absorption spectroscopy. J. Phys. Chem. C 112, 4248–4256 (2008). Figure 1

Enhanced hydrogen evolution reaction activity of FeNi layered double hydroxide modified with Ruthenium nanoparticles at high current density